I think that sometimes the hair styles in anime is used to define the character. or make them stand out...sometimes, For example when ever there is a character who is not very smart and is involved with crime (usually involved with the yakuza ) they tend to have a hair style like this... (image below) but in the case of shiki i honestly dont know why their hair is like that and why their hair ignores gravity.

Yeah, mostly so you can tell one character from another. Unfortunately, as much as I like anime, most of the characters look so similar that all you can do to tell them apart is their hair color in some cases.

What really throws me though is when a family is drawn and the mother has red hair, the father has black hair, and the daughter has blonde hair. Now, I'm no detective but...he might not be the father.